Download Micro-Optics: From High-End to Mass-Market

Micro-Optics
Micro-Optics: From High-End
to Mass-Market
Wafer-based technology now provides high-quality
micro-optics for low-cost consumer products.
Photonics is said to be the most important key technology in the 21st century, some even call the 21st century the
“photon century”. It might be a bit too
early to name a whole century after it,
but indeed, photon-based technology
has much impact on our everyday life at
the beginning of the new century. Chip
manufacturing, lighting, health care and
life-sciences, space, defense, and the
transport and automotive sector rely on
photon-based technology. Photonics is
also supposed to offer novel solutions
where today’s conventional technologies
reach their limits in terms of velocity, capacity and accuracy.
If science and technology deal with small
things, we call this micro or nano: microelectronics, microbiology, micro-optics, nano-optics, nanophotonics – I never heard
the term microphotonics. It seems that the
term micro was already unfashionable when
optics was renamed to photonics. To my
understanding these new labels or tags are
effective to fulfill the common demand for
novelty in research and research funding,
even if the ideas behind are well-known
since long – just politics. For micro-optics,
which we will discuss in detail in the following, most ideas and concepts have been
proposed long time ago. However, novel
microfabrication and wafer-based manufacturing technology that now allows to realize
and commercialize micro-optics in a way
scientists have dreamt for generations.
The Author
Reinhard Voelkel
received his Diploma in
Physics in 1989 and PhD
in 1994 from the University of Erlangen-Nuernberg, Germany, where he worked at the
Applied Optics Institute (Prof. Adolf W.
Lohmann, Prof. Johannes Schwider) on
holographic optical elements for optical
interconnects and backplanes. After his
PhD he worked at the Institute of Microtechnology (Prof. René Dandliker, Prof.
Hans Peter Herzig) at University of Neuchatel, Switzerland, on micro-optics for
biosensors, for optical interconnects, for
photolithography system and miniaturized imaging and camera systems.
History of Micro-Optics
Figure 1: Scheme of a drone fly compound eye as drawn by Robert Hook in
Micrographia, published in 1665.
This journal’s name is “optics & photonics”.
What is the difference between photonics
and optics? The term photonics had been
proposed as a counterpart of electronics in
1967 by Pierre Aigrain, a French scientist.
Using photons instead of electrons to process and transfer signals and information.
Optics, a branch of physics, deals with light
sources and the manipulation of light by
lenses, prisms and mirrors since hundreds
of years. Now, since the turn of the century,
photonics is frequently used as the high
tech term that encompasses, electro-optics,
fiber-optics, lasers, solid-state lighting, material science and optics – wherever light is
involved.
Micro-optics is actually a very old concept
in Nature. Compound eyes first appeared
during the Cambrian period, about 540
million years ago. Still today, similar concepts for compound eyes based on microlens arrays are the most appropriate solution for all insects and crustaceans.
The first micro-optics research projects in
history were much related to the development of microscopy. In the 17th century,
Robert Hooke and Antonie van Leeuwenhoek both developed techniques to manufacture small glass lenses to improve their
microscopes. Hooke melted small filaments
of glass and allowed the surface tension in
the molten glass to form the smooth spherical surfaces required for lenses. These tiny
lenses served as front lens in Hooke’s microscopes and radically improve the resolution
beyond everything seen before. Hooke was
the first to observe plant cells, micro-organisms – and also the microlenses of a fly’s
compound eye. In 1665 he published his
famous book “Micrographia” describing his
groundbreaking observations. The book
was a real bestseller and inspired, amongst
many others, also the Dutch Antonie van
Leeuwenhoek. Leeuwenhoek further im-
36 Optik & Photonik December 2009 No. 4
In 1999 he founded together with Martin
Eisner, Kenneth J. Weible and SUSS MicroTec a joint-venture focusing on research
and manufacturing of micro-optics. In
2002 this spin-off changed its name to
SUSS MicroOptics SA where he serves as
CEO. His main research interests include
scientific and technical aspects associated
with micro-optics and miniaturized imaging systems. Reinhard Voelkel is member
of the German Optical Society (DGaO),
the Swiss Optical Society (SSOM), the European Optical Society (EOS), SPIE and
the Optical Society of America (OSA).
●●
Dr. Reinhard Voelkel
SUSS MicroOptics SA
Jaquet-Droz 7, CH-2000 Neuchâtel,
Switzerland
Phone: +41 32 7205-103
Fax: +41 32 7205-713
E-mail: [email protected]
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Micro-Optics
proved the lens manufacturing technique
and built hemispherical lens for microscopes
achieving typically 50x to 300x resolution.
Recent studies proved that some of Leeuwenhoek’s microscopes were even able to
resolve structures in the micrometer range.
Leeuwenhoek was the first to observe living
bacteria, blood and sperm cells, and therefore is commonly recognized as the father
of microbiology. Small hemispherical lenses,
more “minilenses” than microlenses, were
the key enabling component for revolutionary discoveries and the start of new branch
of biology which influences our life until today. It took almost 200 years until Ernst
Abbe in 1880 managed to manufacture microscopes with higher resolution then reaching the so-called Abbe-limit.
First endoscopes to look inside the “canals and cavities of the human body” were
developed in 1806 by Philip Bozzini in Vienna, Austria. It took another 150 years until the German Karl Storz and researchers at
Olympus in Japan were able to provide fully
developed endoscopes for medical observation and minimal invasive surgery. A very
impressive success story which has much
impact until today. Miniaturized lenses, microlenses, rod lenses or fiber bundles are
the key components for image transport
and illumination in all – now – so-called
classical endoscopes. The future generations of endoscopes will use miniaturized
CMOS image sensors (CIS) and LED or OLED illumination. Microlens imprint lithography (SMILE) and wafer-level packaging
(WLP) are used to manufacture disposable
endoscopes providing high-miniaturization,
excellent imaging quality and low costs.
Micro-optics is also a perfect mean for illumination systems. Fly’s-eye condensers,
also referred as microlens homogenizers, optical integrators and Köhler integrators, were
already used in early slide and film projectors
to improve the uniformity of the illumination
light. In general, a first microlens array splits
an incident light beam into beamlets. These
beamlets are redirected by a second microlens array superimposed in the focal plane­
of a large lens. A uniform illumination is
achieved in the superposition plane. Today,
this microlens-based homogenizer concept is
found in a wide range of products from micro beamers in mobile phones to DUV highend wafer steppers in Semiconductor fabs.
In 1908, Nobel Prize winner Gabriel Lippmann proposed integral photography, a
stereoscopic method using an array of small
camera lenslets. In 1940, another Nobel
Prize winner, Dennis Gabor, invented the
Gabor superlens, intended to replace large
and bulky lenses by a combination of two
thin microlens arrays. Gabor’s other – Noble
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Figure 2: Minia­
turized wafer-level
camera (WLC) for
sensors and dis­po­
sable endoscopes.
German research
project COMIKA
(BMBF-VDI), 2009.
100 μm
CMOS Imager
1 pixel
Figure 3: Microlens on CMOS image sensor to enhance fill-factor.
Prize winning – invention of holography in
1947 and the appearance of first lasers inspired Adolf W. Lohmann in 1966 to invent
the computer generated holography (CGH),
a new concept for design and manufacturing of digital micro-optics which is widely
used until today for laser beam shaping and
many other applications. In 1977, Kenichi
Iga was the first to propose 2-dimensional
arrays of surface emitting semiconductor lasers, now referred as VCSELs. In 1982 he
suggested to build of stacked planar optics,
nothing different than the wafer-level packaging (WLP) of micro-optics.
It is interesting to see how early micro-­
optics was actually invented. Unfortunately,
these inventions were just nice ideas and concepts, but difficult or impossible to realize.
Manufacturing of Micro-Optics
Fortunately, much has evolved since first research groups and university institutes for
micro-optics appeared in the 1980s. Researchers started to investigate suitable
technology for design, simulation, manufacturing and testing of micro-optics. In the
1990s, a variety of novel techniques for micro-optics were invented and implemented.
Consequently, a number of small and medium-size micro-optics companies appeared.
The telecom boom, where microlens arrays
were required for fibre arrays, optical interconnects and optical switches, emphasized
commercialization. Today, microlens arrays
are printed on CMOS image sensors to improve the sensitivity of digital cameras; the
backlight illumination system of LCD displays consists of several sheets of array optics to ensure efficient and uniform illumination; cylindrical microlens arrays enlarge the
viewing angle of screens; microlens arrays
collimate and focus light in high-speed photonic switches; microlens beam homogenizers provide flat-top illumination for all kinds
of high-power lasers; diffractive and refractive micro-optics in Fused Silica and Calcium
Fluoride are key enabling components for
high-end DUV ­lithography systems printing
sub 40 nm structures on 300 mm wafers.
In the first decade of the 21st century
micro-optical components and modules
became enabling key parts for many applications and products. However, micro-optics industry remains still divided into two
groups:
a) Small niche players supplying high-quality key components in low volume, e.g.
for laser, telecom, medical and metrology.
Wafer-based manufacturing technology
adapted from semiconductor in­dustry is
the preferred approach to manufacture
www.optik-photonik.de 37
Micro-Optics
Figure 4: Mask Aligner MA/BA8 Gen3 dedicated for Wafer-Level Camera (WLC)
manufacturing and packaging.
(www.suss.com)
1st MLA
Aperture Stop
Vignetting Stop
Field Stop
Field MLA
2nd MLA
2nd MLA Stop
first
Component
second
Component
Glass
Polymer
this high-quality micro-optics with nanometer accuracy for niche markets.
b) Big companies supplying high-volume
and low-costs micro-optics for the consumer market. Roll-on stamping machines, embossing, casting and other
mass-production techniques are used to
manufacture micro-optics at very low
cost.
In both market segments, the availability of
standardized “off-the-shelf” micro-optics is
very poor. Micro-optics is manufactured on
customer’s request. Micro-optics is often a
difficult-to-implement and exotic solution.
A small supplier base, high prototyping
costs, unwanted diffraction or interference
effects, and difficulties to measure and classify the quality are the major drawbacks.
Image plane
Figure 5: Ultraflat
WLC inspired by
Gabor superlens
and compound
eyes, invented at
Fraunhofer IOF,
Jena, Germany.
[Opt. Exp., Vol 17,
No 18, 2009]
Wafer-Level Cameras (WLC) for
Mobile Phones are the Incitement
for a New Micro-Optics Industry
Today, we see a new area of micro-optics
manufacturing on the horizon. New waferbased manufacturing technology like
­microlens imprint lithography (SMILE) and
wafer-level packaging (WLP) allow to manufacture high-quality micro-optics at very low
costs, and – even more important standardized manufacturing processes will also allow
to manufacture low-volume niche products
in these micro-optics fabs. In the near future,
standardized micro-optics will be available in
high-quality and low-cost, both for highvolume and niche markets.
The driving force behind this recent development is the mobile phone industry
38 Optik & Photonik December 2009 No. 4
and its demand for cheap cameras. When
the first camera phones appeared on the
market in 2002, the camera was just another gimmick with limited value for the
mobile phone user. Today, about 80% of
the mobile phones have a camera inside.
For high-end mobile phones, a camera is an
indispensable part; moreover, such phones
are usually equipped with two types of
cameras, a main or primary Mega-Pixel
camera, used for photography and a sub or
secondary CIF or VGA camera, used for
video. For 2009 the amount of sub-cameras
is estimated to be around 210 millions.
Here costs and size are the major issues.
Such low-cost cameras also serve as primary
camera in budget phones, webcams, security, toys and mobile gaming platforms.
Some years ago, a leading mobile phone
company promoted wafer-level cameras
(WLC) as being the ultimate technology for
mobile phone cameras.
The wafer-level camera (WLC) approach
sounds rather simple: All components are
manufactured on 8’’ wafer by microlens imprint lithography, deposition, lift-off, screen
printing, powder blasting and glues dispensing. The optical wafers are then mounted together with micrometer accuracy in a
mask aligner. The camera wafer stack is
mounted by wafer-level packaging onto a
full CMOS image sensor wafer, or the camera wafer stack is diced into individual optics modules and then mounted onto individual CMOS image sensors. The complete
mobile phone camera, including color filters optics, is manufactured and packaged
on wafer-level using wafer-based technology. Despite WLC technology is still not mature, an estimated 80 million WLC modules
have been delivered in 2009. The expected
growth rate for WLC is so attractive, that
most CMOS camera suppliers are now investigating WLC solutions.
The simplicity of the WLC concept often
leads to the misinterpretation that CMOS
manufacturers just have to use their highly
developed semiconductor technology to
manufacture the 8’’ Opto-Wafers. Compared to CMOS manufacturing, the WLC
with some bulky lenses should be rather simple to do. Unfortunately, these assumptions
are not valid. New manufacturing equipment and processes had to be developed
until technology frontrunners in WLC manufacturing, like Heptagon, Anteryon, Aptina,
Tessera and VisEra, were able to deliver first
WLC camera modules to the market.
Microlens Imprint Lithography
Enabling key technology for WLC is a costefficient manufacturing and packaging
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Micro-Optics
technology for lens, pupil and spacer wafers. The most promising technology is UV
replication technology like microlens imprint lithography (SMILE), where a liquid
polymer is dispensed on the wafer and the
lenses are imprinted by using a transparent
stamp or mold and UV-light for curing. Microlens Imprint Lithography allows the
manufacturing of lens arrays with a submicron lateral accuracy on 8’’ wafer level in
a mask aligner. Accurate wedge error compensation and gap setting are crucial for
microlens imprint lithography.
For microlens imprint lithography the
master tool generation is a critical key technology. In the past, smaller lens arrays are
used to build a larger lens master in a stepand-repeat procedure using a NPS 300
­Nano-Imprint-Stepper. For 8’’ lens masters
with a sag height below 100 µm, high-quality microlens wafers manufactured by resist
reflow and reactive ion etching (RIE) are well
suited. Master lens arrays consisting of aspherical microlenses with profile deviation
below 50nm (rms) and surface roughness
below 2nm (rms) were demonstrated on full
8’’ wafer scale by SUSS MicroOptics, Switzerland. Recently Kaleido Technology, Denmark, presented a first 8’’ diamond milled
lens master in brass, providing spherical, aspherical and free-form lenses with better
than 2 µm lateral position accuracy.
The wafer-scale master wafers are then
transferred into a soft stamp, usually made
of Polydimethylsiloxane (PDMS) by casting.
The PDMS layer serves as a stamp. The polymer lens material must be suitable for high
throughput imprint lithography, reflowable
and must have long-time stability in harsh
environment (heat, humidity and sunlight).
For long time, the choice of suitable lens
material for microlens imprint lithography
was very limited. Front runners in WLC kept
their material a secret. Encouraged by high
demand for microlens imprint lithography,
other suppliers like DELO, Germany and
Asahi Kasei, Japan, are now supplying suitable lens material to the market.
The current hype for wafer-level camera
(WLC) will help to establish a new generation of micro-optics manufacturing companies. These highly specialized 8’’ wafer fabs
will master all micro-optics manufacturing
and packaging technologies. They will also
be able to manufacture most other microoptical components, modules and devices
at very low costs. The 8’’ wafer standard
will even allow to run small volume jobs
with some 10 to 100 wafers only. Similar to
the large semiconductor fabs in Asia, these
fabs will increase the availability and improve costs for micro-optics. Current microoptics manufacturers might focus on proto-
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Figure 6: MO
Exposure Optics,
a new illumination
system for mask
aligners based on
microlens array
homogenizers.
(Photo: www.suss.com)
The Company
SUSS MicroTec AG
SUSS MicroTec Group is a leading supplier of process and test solutions for
­micro-structuring applications with more
than sixty years of engineering expe­
rience. Its solution portfolio covers all
­performance relevant steps for wafer processing ranging from coating, baking,
developing, aligning to wafer bonding
and testing and is complemented by specialized add-ons such as photomasks, Microlens Imprint and Nano-Imprint Lithography tools, micro-optics and wafer-level
test accessories. SUSS MicroTec supports
more than 8,000 installed mask aligners,
coaters, bonders and probe systems with
a global infrastructure for applications
and service. SUSS is headquartered in
Garching near Munich, Germany.
For more information,
visit www.suss.com.
typing and master manufacturing. They
might change to fab-light or even fab-less.
As today’s frontrunners in WLC still have to
handle the rapid growth of WLC demand,
this trend will not happen in the next 1–2
years. However, the low margins in mobile
phone camera business will force them to
open their doors for more profitable other
micro-optics products very soon.
Ultra-flat Micro-Cameras based
on Gabor Superlens Concept
About 10 years ago natural compound eye
concepts inspired micro-optics researchers
to investigate ultra-flat imaging systems by
combining flat layers of microlens arrays and
image sensors [German Patent DE19917890].
Due to fundamental scaling laws in optics, a
very flat camera approach is not possible
with a single objective. Similar to miniaturized vision systems in Nature, multiple lens
channels are used, whereas the image is either superimposed in the image plane or in
a subsequent electronic system.
A recent publication from researchers at
the Fraunhofer IOF in Jena, Germany, now
shows a combination of a Gabor superlens
concept and a superposition approach inspired by the compound eyes of nocturnal
insects.
Microlens Array Homogenizer
for Illumination Systems
In 1893, August Köhler of the Carl Zeiss corporation in Jena, Germany, introduced a new
and revolutionary method for uniform illumination of specimen in an optical microscope.
The Köhler method allows adjusting the size
and the numerical aperture of the object illumination in a microscope independent
from each other. Köhler illumination provides
uniform illumination of the object plane independent of shape, extension and angular
field of the light source. Köhler illumination
was a major milestone in the history of optical microscopy and is still widely used today.
Köhler illumination is also the basic principle
behind laser beam homogenizing.
At the same time different types of fly’seye condensers were invented to improve
the uniformity of the illumination light in
early slide and film projectors. The most effective form of a microlens homogenizer is
a combination of two identical microlens
arrays located at a focal length’s distance of
each other and a subsequent large lens.
www.optik-photonik.de 39
Micro-Optics
Figure 7: A 50-cm
wide yellow laser
beam is coming out
of ESO’s Very Large
Telescope (VLT)
in Chile. The laser
light illuminate
Sodium atoms
located in the
upper atmosphere
at 90 km altitude
and creates an
artificial star. The
laser guide star
(LGS) is analyzed
by Shack-Hartmann
wavefront sensors
and used to opti­
mize the adaptive
optics of the
telescope mirrors.
(Image by Sylvain
Oberti, ESO)
First microlens array splits an incident light
beam into individual beamlets. These
beamlets are then redirected by the second
microlens array superimposed in the focal
plane of the large lens. A uniform illumination, often referred as “flat-top” is achieved
in the superposition plane. It is not authenticated if August Köhler also developed fly’s
eye condensers himself. However, as each
channel of the described microlens homogenizer represents a Köhler illumination system, such homogenizers are also referred
as Köhler integrators. Today, this microlensbased homogenizer concept is found in a
wide range of products from small micro
beamers in mobile phones to high-end DUV
wafer steppers in Semiconductor fabs.
MO Exposure Optics for
Mask Aligners Based on Two
Köhler Integrators
Most recently a novel illumination system
based on two subsequent Köhler integrators has been introduced for mask aligners.
For the new microlens based illumination
concept, a first Köhler integrator is placed
in the focal plane of the lamp ellipsoid. A
second Köhler integrator is placed in the focal – or Fourier – plane of the first integrator. This combination provides both, a uniform light intensity and a uniform angular
spectrum of the illumination light.
The Company
SUSS MicroOptics SA
SUSS MicroOptics is a world-leading
company for micro-optical components
and systems. SUSS MicroOptics comprises profound experience in optical design, engineering, micro-fabrication, microlens imprint lithography, wafer-level
packaging and metrology. SUSS MicroOptics utilize the company’s leading
edge wafer-level processes and equipment to develop and deliver customized
micro-optic solutions, which are available on a variety of substrates, fabricated
on one or both surfaces of a wafer, and in
stacked-wafer forms. Our micro-optical
components, modules and systems meet
highest demands as required for applications in photolithography, optical networking, imaging, metrology, laser, biomedical and medical applications.
SUSS MicroOptics is committed to providing the highest quality components
through leading edge manufacturing
techniques. Combined with a unique
blend of people skills, its innovative advances in technology make us a leader in
its product offerings. SUSS MicroOptics
is part of the SUSS MicroTec Group.
For more information,
visit www.suss.ch.
40 Optik & Photonik December 2009 No. 4
MO Exposure Optics significantly improves the uniformity of the exposure light
(typically ±2 %), decouples the light for
lamp position errors, improves telecentricity, and for the first time in a mask aligner,
allows customized illumination and optical
proximity correction (OPC) for cost efficient
contact and proximity lithography. Customized illumination and OPC, both well
established methods for high-end stepper
lithography, will now also improve process
window and yield for about 2’000 mask
aligners installed in almost every microtechnology and semiconductor cleanroom and
fab worldwide. Again, micro-optics is the
key enabling element.
Array Optics for Testing – Shack
Hartmann Wavefront Sensors
Micro-optics is also a key enabling technology for ophthalmology and metrology. In
1619, Christoph Scheiner, a Jesuit philosopher and astronomer, analyzed the aberrations of the human eye by using two pinholes. The Scheiner’s disk isolates rays,
allowing their aberrated direction of propagation to be traced. In 1900, Scheiner’s test
method was improved by Johannes Franz
Hartmann, a German astrophysicist, and
used to test the performance of the “Great
Refractor”, a 80 cm refracting telescope at
Potsdam, Germany. Hartmann’s method
was to perforate an opaque screen with numerous holes. Each hole acts as an aperture
to isolate a narrow bundle of light rays so
they could be traced to determine any errors in their direction of propagation. Since
rays are perpendicular to the propagating
wavefront, any error in ray direction is also
an error in wavefront slope. Thus Hartmann’s method is commonly referred to a
wavefront sensor.
In the early 1970s, Roland Shack at the
Optical Sciences Center (OSC) in Arizona,
USA, tried to improve the Hartmann screen
test by replacing the pinhole array with an
array of microlenses. At that time, appropriate microlens arrays were not available. Finally, he assigned Ben Platt, a graduate student, to develop the lenslet array for him.
The student developed his own polishing
machine to manufacture first cross-cylindrical microlens arrays suitable for the Shack
Hartmann wavefront sensor. Today, ShackHartmann sensors are widely used in ophthalmology, astronomy, adaptive optics,
optical alignment, and commercial optical
testing. High-quality microlens arrays providing lateral position and beam point accuracy in the nanometer range are the enabling key component.
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim